As the market demands continuously increase for display systems with higher resolution, greater brightness, lower power, lighter weight and more compact size, spatial light modulators having micromirrors and micromirror arrays have blossomed in display applications. FIG. 1 presents a simplified exemplary display system employing a spatial light modulator. In its very basic configuration, the display system comprises light source 102, optical devices (e.g. light pipe 106, condensing lens 108 and projection lens 116), display target 118 and spatial light modulator 114 that further comprises a plurality of micromirror devices (e.g. an array of micromirror devices). Light source 102 (e.g. an arc lamp) emits light through the light integrator/pipe 106 and condensing lens 108 and onto spatial light modulator 114. Each micromirror device (e.g. micromirror device 110 or 112) of spatial light modulator 114 is associated with a pixel of an image or a video frame and is selectively actuated by a controller (e.g. as disclosed in U.S. Pat. No. 6,388,661 issued May 14, 2002 incorporated herein by reference) so as to reflect light from the light source either into (the micromirror at the ON state) or away from (the micromirror at the OFF state) projection optics 116, resulting in an image or a video frame on display target 118 (screen, a viewer's eyes, a photosensitive material, etc.).
It is generally advantageous to drive the micromirrors of a spatial light modulator with as large a voltage as possible. For example, in a spatial light modulator having an array of micromirrors, a large actuation voltage increases the available electrostatic force available to move the micromirrors associated with pixel elements. Greater electrostatic forces provide more operating margin for the micromirrors—increasing yield. Moreover, the electrostatic forces actuate the micromirrors more reliably and robustly over variations in processing and environment. Greater electrostatic forces also allow the hinges of the micromirrors to be made correspondingly stiffer; stiffer hinges may be advantageous since the material films used to fabricate them may be made thicker and therefore less sensitive to process variability, improving yield. Stiffer hinges may also have larger restoration forces to overcome stiction. The pixel switching speed may also be improved by raising the drive voltage to the pixel, allowing higher frame rates, or greater color bit depth to be achieved.
The application of a high-voltage, however, has disadvantages, one of which is charge accumulation in micromirror devices. Referring to FIG. 2, a cross-sectional view of a micromirror device used in the spatial light modulator in FIG. 1 is illustrated therein. The micromirror device comprises mirror plate 134. The mirror plate rotates relative to glass substrate 130 and reflects light traveling through the glass substrate into different directions. The rotation is achieved by establishing an electrostatic field between the mirror plate and electrode 140, which is formed on substrate 132. In most cases, a dielectric layer, such as dielectric layer 138 (e.g. a SiO2 layer and/or a SiNx layer), is deposited around the edges of the electrode for passivation of the electrode. In operation, the mirror plate and the electrode are connected to a voltage source so as to establish a voltage difference between the mirror plate and the electrode. The voltage difference results in an electrostatic force exerted on the mirror plate for driving the mirror plate to rotate. The voltages applied to the mirror plate and the electrode; however, induce charge to accumulate on the surface of the dielectric layers as shown. These charges accumulate during the operation of the micromirror device, and establish an additional electric field between the mirror plate and the electrode. This additional electric field in turn reduces the electric field created by voltage source 142. Consequently, the electrostatic force exerted to the mirror plate is reduced. That is, the voltage difference necessary to rotate the mirror plate to the desired angle is shifted towards higher voltage. In this situation, operation of the micromirrors of the spatial light modulator becomes unreliable.
Therefore, what is needed is a method and apparatus for providing a high voltage between a micromirror plate and the associated electrode while preventing charge accumulation.